Igog] BORDNER—INFLUENCE OF TRACTION ON STEMS 271 
being subjected to a tension of 250%™ at the beginning, increased to 
500%™ seven days later, and to 750%" on March 19. On account 
of the shortness of the stems, which made it impossible to fasten 
them into the apparatus firmly enough to hold a sufficient weight, 
the attempt to determine the breaking strength of these stems 
Was unsatisfactory. The reduced diameter, in that part of the stem 
below the cotyledons as shown in table IX, was due to a collapse of 
the tissue surrounding the stele. The total xylem area and number 
of hard bast elements, determined by methods 4 and 5, show an 
average increase of the former to be 14 per cent. and of the latter 11 
per cent. 
Summary 
An average of the different determinations made in the foregoing 
experiments is given in table X. In all except experiment 9 with 
Vinca major and experiment 13 with Vicia Faba, the average break- 
ing strength in the 246 tests shows an increase in the experimental 
plants over the controls. This must be attributed to a response to 
traction along the longitudinal axis of the stem. The negative results 
in experiment g show that the older part of the stem, where active 
Srowth has ceased, does not respond by increasing its breaking 
strength. The lack of response in experiment 13 with Vicia Faba 
can be attributed to factors which vitiated the influence of tension. 
These conditions are fully explained in experiments 13 and 14. 
Experiments g and 13, therefore, may be left out of consideration. 
The average area of the cell wall of the hard bast elements shows an 
increase in the experimental plants over the controls for Helianthus 
dnuus and Sinapis alba. This determination was not made for 
the other species investigated. The average number of hard bast 
elements shows an increase in the experimental plants over the con- 
_ trols in all the species for which this determination was made, except 
Vinca major. Phaseolus vulgaris and Rubus occidentalis also show 
an increase of total bast area in the experimental plants over the con- 
trols. The average thickness of the hard bast fibers in Vinca major 
as found to be greater for the experimental stems. . 
In the total xylem area determinations, the only decrease was 1n 
Sinapis alba and Rubus occidentalis, in both of which, however, there 
